📚 Year 8 Edexcel Science: Formula & Theorem Quick Reference Handbook | 八年级爱德思科学：公式定理速查手册
This quick reference handbook gathers the essential formulas, equations and principles you will meet in Year 8 Edexcel Science. From speed and density to circuits and chemical equations, mastering these fundamentals will support your problem-solving in physics, chemistry and biology. Keep it handy for homework, revision and end-of-topic tests.
这本速查手册汇集了爱德思八年级科学中你会遇到的核心公式、方程式与定理。从速度、密度到电路和化学方程式，掌握这些基础将助你轻松解决物理、化学和生物中的问题。适合用于家庭作业、复习和单元测验。
1. Speed and Motion | 速度与运动
Speed tells us how fast an object moves. It is the distance travelled per unit of time. The average speed of an object can be calculated using the formula below.
速度告诉我们物体运动的快慢，它是单位时间内通过的距离。物体的平均速度可用下面的公式计算。
speed = distance ÷ time
In symbols this is often written as v = d / t or s = d / t, where d represents distance, t represents time and v (or s) represents speed. The SI unit of speed is metre per second (m/s), but you will also see kilometres per hour (km/h).
用符号通常表示为 v = d ÷ t 或 s = d ÷ t，其中 d 代表距离，t 代表时间，v（或 s）代表速度。速度的国际单位是米每秒（m/s），也会用到千米每小时（km/h）。
ight line at a steady rate, the speed is constant; if it changes, you can still calculate the average speed over the whole journey.
示例：一名自行车手在 20 秒内骑行 300 米，他的平均速度是 300 ÷ 20 = 15 m/s。如果沿直线匀速运动，速度为恒量；若速度变化，仍可算出整个行程的平均速度。
2. Density | 密度
Density measures how much mass is packed into a given volume. It helps to explain why some objects float while others sink. The density of a material is found by dividing its mass by its volume.
密度反映单位体积内所含质量的大小，帮助解释为什么有些物体浮起而有些下沉。材料的密度由其质量除以体积得出。
density = mass ÷ volume
In symbols: ρ = m / V, where ρ (the Greek letter rho) is density, m is mass and V is volume. The standard unit is kilogram per cubic metre (kg/m³), but grams per cubic centimetre (g/cm³) are also commonly used.
符号表示为：ρ = m ÷ V，其中 ρ（希腊字母 rho）为密度，m 为质量，V 为体积。常用单位是千克每立方米（kg/m³），也常用克每立方厘米（g/cm³）。
= 5 g/cm³. Water has a density of 1 g/cm³; objects with density less than this will float.
示例：一块金属质量为 500 克，体积为 100 立方厘米，其密度为 500 ÷ 100 = 5 g/cm³。水的密度为 1 g/cm³，密度小于此值的物体会浮起。
3. Pressure | 压强
Pressure is a measure of how concentrated a force is over a surface. A large force acting on a small area produces a high pressure. The relationship is given by the pressure formula.
压强衡量力在表面上的集中程度。作用在小面积上的大力会产生高压强。压强公式揭示了这一关系。
pressure = force ÷ area
In symbols: p = F / A he area in square metres (m²). The resulting unit is the pascal (Pa), where 1 Pa = 1 N/m².
符号表示为：p = F ÷ A，其中 p 为压强，F 为力（牛顿，N），A 为面积（平方米，m²）。计算出的单位是帕斯卡（Pa），1 Pa = 1 N/m²。
To reduce pressure, you can either decrease the force or increase the area. For example, snowshoes have a large surface area to stop you sinking into soft snow by lowering the pressure on the ground.
减小压强可减小力或增大面积。例如，雪鞋具有较大的表面积，通过减小对地面的压强防止陷入松软的雪中。
4. Moments | 力矩
A moment is the turning effect of a force applied at a distance from a pivot. It explains how levers, seesaws and spanners work. The size of a moment depends on two factors: the size of the force and the perpendicular distance from the pivot to the line of action of the force.
力矩是力在距支点一定距离处产生的转动效应，解释了杠杆、跷跷板和扳手的原理。力矩的大小取决于两个因素：力的大小和支点到力作用线的垂直距离。
moment = force × perpendicular distance
In symbols: M = F × d, where M is the moment in newton-metres (Nm), F is the force in newtons (N), and d is the perpendicular distance in metres (m). Always make sure the distance is measured at right angles to the force.
符号表示为：M = F × d，其中 M 为力矩（牛顿·米，Nm），F 为力（牛顿），d 为垂直距离（米）。确保距离是与力垂直的方向量取的。
When an object is balanced, the total clockwise moment equals the total anticlockwise moment. This is called the principle of moments.
当物体平衡时，顺时针力矩之和等于逆时针力矩之和，这就是力矩原理。
5. Work Done | 做功
In science, 'work' is done when a force moves an object in the direction of the force. Work transfers energy from one store to another. The amount of work done is calculated using the equation below.
科学中，“做功”指的是力使物体沿力的方向移动。做功将能量从一个储存转移到另一个。做功的大小用以下公式计算。
work done = force × distance
In symbols: W = F × s (or W = F × d), where W is work done in joules (J), F is the force in newtons (N) and s or d is the distance moved in metres (m). One joule is the work done when a force of 1 newton moves an object 1 metre.
符号表示为：W = F × s（或 W = F × d），其中 W 为功（焦耳，J），F 为力（牛顿），s 或 d 为移动的距离（米）。1 焦耳就是 1 牛顿的力使物体移动 1 米所做的功。
through a height of 2 m requires 20 × 2 = 40 J of work. The energy transferred to the box's gravitational potential store also equals 40 J, assuming no energy is wasted.
示例：用 20 N 的力将盒子提升 2 米需做 20 × 2 = 40 J 的功。假设无能量浪费，转移到盒子重力势能储存的能量也是 40 J。
6. Power and Efficiency | 功率与效率
Power is the rate at which work is done or energy is transferred. It tells you how quickly energy is converted from one form to another. The formula links power to work or energy and time.
功率是做功或能量转移的速率，表明能量从一种形式转换为另一种形式有多快。公式将功率与功（或能量）和时间联系起来。
power = work done ÷ time and power = energy transferred ÷ time
In symbols: P = W / t or P = E / t, with P measured in watts (W), work or energy in joules (J) and time in seconds (s). One watt equals one joule per second.
符号表示为：P = W ÷ t 或 P = E ÷ t，P 的单位是瓦特（W），功或能量单位为焦耳（J），时间单位为秒（s）。1 瓦特等于 1 焦耳每秒。
Efficiency describes how well a device converts input energy into useful output energy. It is often expressed as a percentage.
效率描述设备将输入能量转化为有用输出能量的程度，通常以百分数表示。
efficiency = (useful energy output ÷ total energy input) × 100%
No device can be 100% efficient because some energy is always lost as heat, sound or light that is not useful.
没有设备的效率能达到 100%，因为总有一些能量以无用的热、声或光形式散失。
7. Ohm’s Law | 欧姆定律
In electrical circuits, current, voltage and resistance are linked by Ohm’s Law. The law states that the current flowing through a resistor at constant temperature is directly proportional to the voltage across it.
在电路中，电流、电压和电阻通过欧姆定律联系在一起。定律指出，温度恒定时，流经电阻的电流与其两端的电压成正比。
voltage = current × resistance
In symbols: V = I × R, where V is the potential difference in volts (V), I is the current in amperes (A) and R is the resistance in ohms (Ω). This equation can be rearranged: I = V / R and R = V / I.
符号表示为：V = I × R，其中 V 为电压（伏特，V），I 为电流（安培，A），R 为电阻（欧姆，Ω）。此公式可变形为：I = V ÷ R 以及 R = V ÷ I。
resistance of 10 Ω is connected to a 5 V supply. The current through it is 5 ÷ 10 = 0.5 A.
示例：一个电阻为 10 Ω 的灯泡连接到 5 V 电源，流过它的电流为 5 ÷ 10 = 0.5 A。
8. Series and Parallel Circuits | 串联与并联电路规律
Components can be connected in series or in parallel, and the rules for current, voltage and resistance differ in each case. Mastering these rules is essential for analysing Year 8 circuits.
元件可串联或并联连接，电流、电压和电阻的规律各不相同。掌握这些规则对分析八年级电路至关重要。
Series circuits:
串联电路：
The current is the same everywhere: Itotal = I₁ = I₂ = I₃
各处电流相同：Itotal = I₁ = I₂ = I₃
The supply voltage is shared across the components: Vtotal = V₁ + V₂ + V₃
电源电压在各元件间分配：Vtotal = V₁ + V₂ + V₃
The total resistance is the sum of individual resistances: Rtotal = R₁ + R₂ + R₃
总电阻等于各电阻之和：Rtotal = R₁ + R₂ + R₃
Parallel circuits:
并联电路：
The current from the source splits between branches: Itotal = I₁ + I₂ + I₃
源电流在各支路间分流：Itotal = I₁ + I₂ + I₃
The voltage across each branch is the same as the supply voltage: V₁ = V₂ = Vsupply
每条支路两端电压等于电源电压：V₁ = V₂ = Vsupply
The total resistance is found using the reciprocal formula: 1/Rtotal = 1/R₁ + 1/R₂ (for two resistors). The overall resistance is always smaller than the smallest individual resistance.
总电阻用倒数公式计算：1/Rtotal = 1/R₁ + 1/R₂（两个电阻时）。总电阻总是小于最小的单个电阻。
9. Magnification | 放大率
When using a microscope or hand lens, the image you see is often larger than the actual specimen. Magnification tells you how many times larger the image is compared with the real object.
使用显微镜或放大镜时，看到的图像通常比实际标本大。放大率告诉你图像是实际物体的多少倍。
magnification = image size ÷ actual size
In symbols: M = I / A or simply using the words. Both image size and actual size must be in the same units (e.g. millimetres). Magnification has no units; it is just a ratio.
符号表示为：M = I ÷ A 或直接用文字表示。图像大小和实际大小必须使用相同单位（如毫米）。放大率没有单位，只是一个比值。
us as 15 mm across, but its real diameter is 0.015 mm. Magnification = 15 ÷ 0.015 = 1000×.
示例：一张细胞照片显示细胞核直径为 15 mm，而实际直径为 0.015 mm，放大率为 15 ÷ 0.015 = 1000 倍。
10. Chemical Formulae and Equations | 化学式与方程式
Chemical formulae show the elements present in a compound and the ratio of atoms. The subscript number after each symbol tells you how many atoms of that element are in one unit of the compound. If there is no subscript, it means one atom.
化学式显示化合物中存在的元素以及原子的比例。元素符号后的下标数字表示该化合物一个单元中含有该元素的原子数。无下标则表示一个原子。
Examples: Water H₂O – two hydrogen atoms, one oxygen atom. Carbon dioxide CO₂ – one carbon atom, two oxygen atoms. Methane CH₄ – one carbon, four hydrogen atoms.
示例：水 H₂O——两个氢原子、一个氧原子；二氧化碳 CO₂——一个碳原子、两个氧原子；甲烷 CH₄——一个碳原子、四个氢原子。
eactants go on the left and products on the right, linked by an arrow (→). The arrow means 'react to form'.
化学方程式用符号和化学式描述化学反应。反应物写在左边，生成物写在右边，中间用箭头（→）连接，箭头表示“反应生成”。
For example, the word equation 'hydrogen + oxygen → water' becomes: H₂ + O₂ → H₂O. This equation is not yet balanced.
例如，文字表达式“氢气 + 氧气 → 水”可写为：H₂ + O₂ → H₂O。该式还未配平。
11. Balancing Equations | 配平方程式
Because of the law of conservation of mass, atoms cannot be created or destroyed in a chemical reaction. Therefore, a correct chemical equation must have the same number of each type of atom on both sides. This is achieved by placing large numbers (coefficients) in front of the formulae.
根据质量守恒定律，化学反应中原子不能被创造或消灭。因此，正确的化学方程式必须在两边拥有相同数量的各类原子。这通过在化学式前面放置大的数字（系数）来实现。
Let's balance H₂ + O₂ → H₂O. Count atoms:
让我们配平 H₂ + O₂ → H₂O。数原子：
Left: H = 2, O = 2 | Right: H = 2, O = 1 |
Published by TutorHao | Year 8 Science Revision Series | aleveler.com
